JP7764699B2 - Manufacturing method of surface-coated copper filler - Google Patents

Description

The present invention relates to a method for producing a surface-coated copper filler, the surface-coated copper filler obtained by the production method, and a conductive composition containing the same.

Conductive compositions have traditionally been used in the field of electronic materials for applications such as internal electrodes for multi-layer ceramic capacitors (MLCCs), through-hole bonding materials for printed circuit boards, and materials for forming lead wiring for touch panels.

The conductive composition referred to here is a fluid compound, such as silver paste. This conductive composition can be used to draw a pattern using screen printing or inkjet printing (hereinafter referred to as IJ printing), and then a conductive cured product can be formed by applying light or heat. Silver is often used as the conductive metal in conductive compositions because of its excellent oxidation resistance and low volume resistivity. However, silver has issues such as being expensive and prone to migration. Therefore, in recent years, the use of copper in conductive compositions has been considered, as it has the second lowest volume resistivity after silver, is inexpensive, and has excellent migration resistance.

Commercially available conductive copper particles with particle sizes between 100 nm and 20 μm used in conductive compositions are generally coated with a fatty acid to improve the dispersibility and oxidation resistance of the copper particle powder. However, coating with a fatty acid alone does not provide sufficient oxidation resistance, and the conductive copper particles oxidize due to the high oxidizability of copper, resulting in copper oxide on part of the surface.

Patent Document 1 discloses a method for producing surface-coated copper filler, in which copper particles are pretreated by washing them with an organic solvent or acid to prevent oxidation of the copper particle surface, and then a first coating layer of a specific amine compound and a second coating layer of a specific aliphatic monocarboxylic acid are formed. Patent Document 2 also discloses a method for removing impurities and oxides from the surface of copper particles using an SUS reaction vessel commonly used in the production of chemical products. This method involves adding copper particles to a solution containing a specific ammonium halide derivative, which is less corrosive than the acids such as hydrochloric acid used in the pretreatment of Patent Document 1, to deposit copper chloride on the copper particle surface, and then adding these copper chloride-deposited copper particles to a solution containing a specific amine compound to remove the copper chloride and form a coating layer of the specific amine compound on the copper particle surface.

International Publication No. 2016/199811 Japanese Patent Application Laid-Open No. 2018-165382

The inventors believed that combining the methods described in Patent Documents 1 and 2 would provide the benefits of both methods, and therefore investigated forming a coating layer of a specific amine compound on the surface of copper particles according to the method disclosed in Patent Document 2, and then forming a coating layer of a specific aliphatic monocarboxylic acid on that coating layer, as disclosed in Patent Document 1. However, although the surface-coated copper filler obtained in this manner has superior oxidation resistance compared to that obtained by the method described in Patent Document 1, it was found that some of the copper particles may aggregate when the amine compound coating layer is formed.

When copper particles aggregate in this way, the aggregated copper particles have a lower bulk density and occupy a larger volume in the solution. Therefore, for example, if the copper particles are allowed to settle, the supernatant liquid is removed by decantation, and then the treatment liquid is replaced by adding new treatment liquid, the replacement rate of the treatment liquid will be poor, making it necessary to replace the treatment liquid more frequently, which may reduce production efficiency.

Surface-coated copper fillers are sometimes applied to conductive compositions, for example. Generally, these conductive compositions are often used to form wiring patterns by screen printing. Therefore, when conductive compositions use surface-coated copper fillers containing agglomerated copper particles, the agglomerates often clog the mesh of the screen, causing defects in the printed wiring. This has been a factor preventing the use of conductive compositions using surface-coated copper fillers in forming wiring patterns.

The objective of the present invention is to provide a method for producing surface-coated copper filler that has fewer agglomerated copper particles and has the same excellent oxidation resistance as conventional fillers.

The present inventors conducted extensive research in light of the above-mentioned problems. As a result, they discovered that, unlike Patent Documents 1 and 2, when forming a first coating layer of the specific amine compound described in Patent Documents 1 and 2 on the surface of copper particles, by immersing the copper particles in a mixed solution in which the aforementioned specific amine compound and a specific compound coexist in a predetermined mixing ratio, it is possible to produce a surface-coated copper filler with little copper particle aggregation and good oxidation resistance, which led to the completion of the present invention. In other words, the gist of the present invention is as follows.

[1] (A) Immersing copper particles in a solution containing 100 parts by mass of an amine compound represented by formula (1) and 0.01 to 30 parts by mass of a metal halide or ammonium halide derivative.

[In formula (1), m is an integer from 0 to 3, and n is an integer from 0 to 2. When n = 0, m is any of 0 to 3. When n = 1 or n = 2, m is any of 1 to 3.]

(B) a step of immersing the copper particles that have been subjected to the step (A) in a solution containing an aliphatic monocarboxylic acid having 8 to 24 carbon atoms;
A method for producing a surface-coated copper filler, comprising:

[2] The method for producing a surface-coated copper filler according to the above item [1], wherein the halogen atoms contained in the metal halide and the ammonium halide derivative are chlorine atoms.
[3] The method for producing a surface-coated copper filler according to the above item [1] or [2], wherein the ammonium halide derivative is selected from ammonium halide derivatives represented by formula (2):

[In formula (2), a is an integer from 0 to 3, b is an integer from 0 to 3, and a + b = 3.]

[4] A surface-coated copper filler obtained by the above-mentioned production method.
[5] A conductive composition containing 10 to 95 mass % of the surface-coated copper filler.

The method for producing surface-coated copper filler of the present invention can provide a method for producing surface-coated copper filler with fewer agglomerated copper particles and with oxidation resistance equivalent to that of conventional methods.

Embodiments of the present invention are described in detail below.

<Copper particles as raw material>
First, copper particles that are the raw material for the manufacturing method according to the embodiment of the present invention will be described. Hereinafter, when referring to copper particles, unless otherwise specified, it refers to raw copper particles.

Examples of copper particles include known copper particles commonly used in copper pastes and copper inks. They may be spherical, plate-like, dendritic, rod-like, or fibrous, and may be amorphous, such as hollow or porous. Furthermore, they may be core-shell particles, in which the shell is copper and the core is made of a material other than copper.

The average particle size of the copper particles is not particularly limited. However, when used for conductive compositions, for example, the average particle size of the copper particles is controlled so that the conductive composition can be printed using various printing methods, such as inkjet printing and screen printing. Specifically, the average particle size is preferably 5 nm to 20 μm. To suppress particle self-aggregation, suppress oxidation by increasing surface area, or to print fine wiring of 100 μm or less, the average particle size is preferably 10 nm to 10 μm. When used for conductive compositions for screen printing, which has excellent continuous printability, the average particle size is preferably 100 nm to 10 μm. Furthermore, copper particles may be of one type, or a mixture of copper particles with different shapes and average particle sizes may be used. In other words, a single type of homogeneous powder having a certain particle size distribution may be used, or multiple types of such powders may be used in combination. Examples of combinations of different powder groups include combinations of groups of copper particles with different shapes, and combinations of groups with particle size distributions having different average particle sizes.

The average particle size of copper particles refers to the value obtained by arithmetically averaging the Feret diameters of 100 particles randomly selected from microscopic images obtained by observation with a transmission electron microscope or scanning electron microscope.

<Surface-coated copper filler>
The surface-coated copper filler according to an embodiment of the present invention comprises copper particles, a first coating layer of an amine compound represented by formula (1) (hereinafter, sometimes simply referred to as the "amine compound") that is chemically and/or physically bonded to copper atoms on the surface of the copper particles, and a second coating layer on the first coating layer of an aliphatic monocarboxylic acid having 8 to 24 carbon atoms that is chemically bonded to the amine compound.

[In formula (1), m is an integer from 0 to 3, and n is an integer from 0 to 2. When n = 0, m is any of 0 to 3. When n = 1 or n = 2, m is any of 1 to 3.]

The first coating layer is a layer of an amine compound that is chemically and/or physically bonded to and adsorbed to copper atoms on the surface of the copper particles. From the standpoint of oxidation resistance, it would be ideal for the amine compound to uniformly coat the copper particle surface in the form of a monomolecular film. However, in practice, it is difficult to achieve such an ideal state. Therefore, there may be some areas on the copper surface where the amine compound is not adsorbed, or there may be areas where two or more molecules are adsorbed in layers.

Therefore, the first coating layer includes not only a layer in which the amine compound uniformly coats the copper particle surface, but also a coating layer in which there is a portion of the copper particle surface to which the amine compound has not yet been adsorbed.

The fact that the amine compound is adsorbed onto the copper particle surface to form the first coating layer can be confirmed by measuring the infrared absorption (IR) spectrum of the copper particle surface (hereinafter sometimes referred to as "IR measurement") as described below.

Here, "chemically bonded and adsorbed" means that the amine compound forms bonds with copper atoms on the copper particle surface through electrostatic interaction, and is adsorbed to the copper particle surface. Here, electrostatic interaction refers to hydrogen bonds, ionic interactions (ionic bonds), etc. Furthermore, "physically bonded and adsorbed" means that the amine compound is adsorbed to the copper surface through physical adsorption due to van der Waals forces. In particular, the amino groups contained in the amine compound have high electron-donating properties, and it is thought that bonds are formed by the amino groups forming coordination bonds with copper. Therefore, it is thought that the amine compound is primarily adsorbed to the copper particle surface through chemical bonds due to electrostatic interaction, forming the first coating layer. However, some adsorption through physical bonds may also occur.

In addition, there may be portions where two or more amine compound molecules are bonded to each other, for example, by hydrogen bonding, etc., to form a stack.

The second coating layer is a layer laminated on the first coating layer and is a layer of an aliphatic monocarboxylic acid having 8 to 24 carbon atoms (hereinafter sometimes simply referred to as "aliphatic monocarboxylic acid") that is chemically bonded to the amine compound of the first coating layer. It is preferable that the aliphatic monocarboxylic acid uniformly coat the first coating layer on a monomolecular film.

Here, chemical bonding means that the carboxyl group of the aliphatic monocarboxylic acid and the amino group of the amine compound are bonded through electrostatic interaction. Here, electrostatic interaction refers to hydrogen bonding, ionic interaction (ionic bonding), etc. Therefore, the second coating layer can also be said to be a layer of aliphatic monocarboxylic acid that is bonded to the amine compound of the first coating layer through electrostatic interaction. Ideally, the amine compound of the first coating layer and the aliphatic monocarboxylic acid react in a 1:1 ratio to form the second coating layer, but in practice, it is difficult to achieve such an ideal state. Therefore, there may be some amine compound in the first coating layer that is not bonded to the aliphatic monocarboxylic acid, and there may also be portions in the second coating layer where two or more molecules of aliphatic monocarboxylic acid are adsorbed in layers due to physical adsorption or other means.

Therefore, the second coating layer, like the first coating layer, includes not only a layer in which the aliphatic monocarboxylic acid uniformly coats the first coating layer, but also a coating layer formed so that there are portions where the aliphatic monocarboxylic acid is not bonded to the amine compound.

As with the first coating layer, whether the aliphatic monocarboxylic acid has been adsorbed to form the second coating layer can be confirmed by IR measurement of the copper particle surface, as described below.

Furthermore, if there is a portion of the copper particle surface to which the amine compound is not bonded, there may be a portion on the copper particle surface where the aliphatic monocarboxylic acid is directly adsorbed, and such surface-coated copper fillers are also within the scope of the present invention.

The amine compound that forms the first coating layer is an amine compound represented by formula (1). Because the amino group in this amine compound has reducing properties, it has the effect of removing oxides from the copper particle surface and inhibiting oxidation. There are no particular limitations on the amine compound represented by formula (1) that can be used. Specific examples include hydrazine, methylenediamine, ethylenediamine, 1,3-propanediamine, dimethylenetriamine, trimethylenetetramine, diethylenetriamine, triethylenetetramine, dipropylenetriamine, and tripropylenetetramine. The first coating layer may be formed using one or more of these amine compounds.

If the value of m in formula (1) is 4 or greater, the number of amino groups per unit area on the copper particle surface that contribute to chemical bonding and reduction decreases, which may result in insufficient desired oxidation resistance and increased surface oxidation of the copper. Furthermore, if n in formula (1) is 3 or greater, the molecular chain becomes too long, causing steric hindrance with adjacent amine compounds during coating, preventing sufficient coating of the copper particle surface. Again, this may result in insufficient desired oxidation resistance and increased surface oxidation of the copper.

The aliphatic monocarboxylic acid having 8 to 24 carbon atoms that forms the second coating layer includes linear saturated aliphatic monocarboxylic acids having 8 to 24 carbon atoms, linear unsaturated aliphatic monocarboxylic acids having 8 to 24 carbon atoms, branched saturated aliphatic monocarboxylic acids having 8 to 24 carbon atoms, and branched unsaturated saturated aliphatic monocarboxylic acids having 8 to 24 carbon atoms. Specific examples of linear saturated aliphatic monocarboxylic acids having 8 to 24 carbon atoms include caprylic acid, pelargonic acid, capric acid, undecylic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, nonadecanoic acid, and arachidic acid. Examples of linear unsaturated aliphatic monocarboxylic acids having 8 to 24 carbon atoms include myristoleic acid, palmitoleic acid, petroselinic acid, and oleic acid. Examples of branched saturated aliphatic monocarboxylic acids having 8 to 24 carbon atoms include 2-ethylhexanoic acid. The above aliphatic monocarboxylic acids may be used alone or in combination.

If the number of carbon atoms is 7 or less, the alkyl chain length will be short, which may result in poor dispersibility of the surface-coated copper filler. Furthermore, if the number of carbon atoms is 25 or more, the hydrophobicity of the aliphatic monocarboxylic acid will increase, resulting in high compatibility with the binder. When a conductive composition is formed, the aliphatic monocarboxylic acid will be more likely to leave the second coating layer and leach into the binder.

From the viewpoint of further improving the dispersibility of the surface-coated copper filler in the conductive composition and reducing the amount of free aliphatic monocarboxylic acid in the conductive composition, aliphatic monocarboxylic acids having 10 to 18 carbon atoms are preferred. Furthermore, linear saturated aliphatic monocarboxylic acids are more likely to form a closely packed structure than branched or unsaturated aliphatic monocarboxylic acids, resulting in a coating with fewer voids, so it is more preferable to use linear saturated aliphatic monocarboxylic acids having 10 to 18 carbon atoms for the coating.

As described above, the surface-coated copper filler comprises copper particles, a first coating layer of an amine compound that coats the surface of the copper particles, and a second coating layer of an aliphatic monocarboxylic acid that is formed on the first coating layer.

Furthermore, the amine compound contained in the first coating layer has a higher coordination ability with copper atoms than aliphatic monocarboxylic acids due to the effect of the lone electron pair of the nitrogen in the amino group, and is therefore more strongly bonded to the copper particle surface than aliphatic monocarboxylic acids, resulting in a higher surface coverage than aliphatic monocarboxylic acids. Furthermore, the amine compound is likely to form bonds with aliphatic monocarboxylic acids through electrostatic interactions. Here, electrostatic interactions refer to hydrogen bonds and ionic interactions, as described above. Therefore, by coating the copper particle surface with an amine compound with a high surface coverage and then further coating the outer surface with aliphatic monocarboxylic acids, the aliphatic monocarboxylic acids can be coated on the copper particles at a higher surface coverage than by directly coating the copper particles with the aliphatic monocarboxylic acids. Therefore, due to the oxidation-inhibiting effect of the amine compound and the high coverage of the aliphatic monocarboxylic acids, the surface-coated copper filler has higher oxidation resistance than copper fillers coated only with aliphatic monocarboxylic acids.

As mentioned above, it is believed that the carboxyl group of the aliphatic monocarboxylic acid bonds with the amino group of the amine compound through electrostatic interaction. In other words, it is believed that the hydrophilic carboxyl group faces the first coating layer of the amine compound, and the hydrophobic saturated or unsaturated aliphatic hydrocarbon group faces outward, forming a second coating layer. Therefore, surface-coated copper filler having a second coating layer of aliphatic monocarboxylic acid can suppress copper filler aggregation and detachment of the amine compound more effectively than copper filler in which copper particles are coated only with an amine compound.

As mentioned above, it is possible to confirm that a surface-coated copper filler has a first coating layer of an amine compound and a second coating layer of an aliphatic monocarboxylic acid by measuring the IR spectrum of the surface-coated copper filler. As an example, the results of measuring the IR spectrum of a surface-coated copper filler having a first coating layer of diethylenetriamine and a second coating layer of lauric acid are as follows:

When diethylenetriamine was measured alone, the N-H bending vibration peak appeared at 1598 cm ^-1 , whereas the N-H bending vibration peak observed in the surface-coated copper filler was shifted to 1576 cm ^-1 , a lower wavenumber. This indicates that diethylenetriamine is present in a coordinated state on the copper particle surface. Furthermore, it was found that the C=O stretching vibration peak of lauric acid was not observed at 1700 cm ^-1 , and the carboxylate anion ( ^-COO- ) peak was observed at 1417 cm ^-1 . This indicates that lauric acid is present in a bond with the amine compound through electrostatic interaction.

<Method for manufacturing surface-coated copper filler>
A method for producing a surface-coated copper filler according to an embodiment of the present invention will now be described. The production method can include, for example, the following steps (A) to (C).

(1) Process (A)
Step (A) involves immersing copper particles in a solution containing 100 parts by mass of the amine compound represented by formula (1) and 0.01 to 10 parts by mass of a metal halide or an ammonium halide derivative (hereinafter, both may be collectively referred to as "halogen compound"). In this step, impurities on the copper particle surface can be removed and the copper particle surface can be coated with the amine compound while the copper particles are dispersed. Furthermore, unlike the case of Patent Document 2, by mixing the amine compound and the halogen compound at a predetermined mass ratio with the copper particles, it is possible to suppress the occurrence of copper particle aggregation. Note that "impurities on the copper particle surface" refers to components attached to the surface of the copper particles, such as copper salts, dispersants, and copper oxides, which are derived from the production of the copper particles.

Specifically, copper particles are added to a solution containing both an amine compound and a halogen compound in a predetermined ratio to form mixture a, and stirring this mixture a simultaneously prevents the copper particles from agglomerating and removes impurities from the surface of the copper particles, forming a first coating layer of the amine compound on the copper particle surface. The stirring method is not particularly limited, and any stirring method can be used, as long as the copper particles are stirred sufficiently to bring the amine compound and halogen compound into contact with each other. A common stirring method using a known stirrer, such as a paddle stirrer or line mixer, can be used. For copper particles with a small average particle size (approximately 5-10 μm), from the perspective of preventing agglomeration, an alternative method is to mix the copper particles with an amine compound solution that does not contain a halogen compound, stir for a certain period of time, thoroughly disperse the copper particles in the solution, and then add the halogen compound.

Ideally, a first coating layer is formed in which the amine compound uniformly coats the copper particle surface in the form of a monomolecular film, and it is preferable to form a good first coating layer that is as close to ideal as possible. Therefore, the mixing ratio of copper particles to amine compound in step (A) is preferably a ratio that is suitable for forming this good first coating layer.

Specifically, although this depends on the particle size of the copper particles, the amount of amine compound is preferably 1 to 500 parts by mass per 100 parts by mass of copper particles, and from the viewpoint of preventing free amine compound from remaining in the surface-coated copper filler, 10 to 350 parts by mass is more preferred, 30 to 250 parts by mass is even more preferred, and 50 to 150 parts by mass is particularly preferred. Since the smaller the particle size of the copper particles, the larger the surface area per unit mass, it is preferable to mix a larger amount of amine compound with smaller particle sizes.

The mixing ratio of the halogen compound also depends on the particle size of the copper particles, but is generally 0.01 to 30 parts by mass, preferably 0.01 to 25 parts by mass, more preferably 0.02 to 10 parts by mass, and even more preferably 0.05 to 5 parts by mass, of the halogen compound per 100 parts by mass of the amine compound. If the halogen compound is more than 30 parts by mass, the halogen compound may remain in the surface-coated copper filler and act as an impurity, causing an adverse effect when the composition is made. If the amount is less than 0.01 part by mass, the amount is too small to achieve sufficient impurity removal and oxidation resistance.

The halogen compound used is a metal halide or an ammonium halide derivative. These are preferred because they inhibit the aggregation of copper particles in the presence of an amine compound and, from the viewpoint of industrial usefulness, are less corrosive to SUS reactors. Furthermore, the halogen atom contained in the halogen compound may be any of bromine, fluorine, and iodine atoms, but chlorine atoms are particularly preferred from the viewpoints of their oxidation resistance and availability. Furthermore, from the viewpoint of industrial availability, the metal halide is preferably at least one selected from copper chloride, iron chloride, potassium chloride, calcium chloride, sodium chloride, zinc chloride, magnesium chloride, and lithium chloride, and the ammonium halide derivative is preferably at least one selected from ammonium halide derivatives represented by formula (2) (hereinafter sometimes simply referred to as "ammonium halide derivatives").

[In formula (2), a is an integer from 0 to 3, b is an integer from 0 to 3, and a + b = 3.]

Specific examples of ammonium halide derivatives represented by formula (2) include ammonium chloride (a: 3, b: 0), methylammonium chloride (a: 2, b: 1), dimethylammonium chloride (a: 1, b: 2), and trimethylammonium chloride (a: 0, b: 3).

Among the above halogen compounds, ammonium halide derivatives are more preferred when the copper particle surface has a large amount of oxides or fatty acids or other deposits, and among these, ammonium chloride is particularly preferred from the standpoint of its impurity removal effect and availability.

The solvent for dissolving the amine compound and halogen compound is not particularly limited as long as it dissolves the amine compound, has good wettability with copper particles, and does not react with the amine compound and aliphatic monocarboxylic acid. Preferably, the solvent contains one or more selected from alcohols, ketones, ethers, nitriles, sulfoxides, pyrrolidones, and water. Specific examples of alcohols include methanol, ethanol, 1-propanol, isopropyl alcohol, 1-butanol, 2-butanol, 1-pentanol, tert-amyl alcohol, ethylene glycol, butoxyethanol, methoxyethanol, ethoxyethanol, propylene glycol, propylene glycol monomethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether. Examples of ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone. Examples of ethers include diethyl ether and dibutyl ether. Examples of nitriles include acetonitrile, propionitrile, butyronitrile, and isobutyronitrile. Examples of sulfoxides include dimethyl sulfoxide. Examples of pyrrolidones include 1-methyl-2-pyrrolidone.

The treatment temperature for forming the first coating layer should be a temperature at which the amine compound coating progresses without solidifying the solution, and should be a temperature at which copper oxidation is minimized. Specifically, a temperature in the range of -10 to 120°C is preferred. A temperature in the range of 30 to 100°C is even more preferred, as it allows for a higher coating rate and minimizes oxidation.

There are no particular restrictions on the treatment time, but 5 minutes to 10 hours is preferred, as this allows for sufficient coating of the amine compound and prevents the salt of the amine compound and carbon dioxide formed by the incorporation of carbon dioxide from the atmosphere from remaining as an impurity in the surface-coated copper filler. 5 minutes to 3 hours is even more preferred from the standpoint of production costs.

In addition, since this makes it possible to suppress salt formation between the amine compound and atmospheric carbon dioxide and oxidation of the copper, it is preferable to carry out step (A) in an inert gas atmosphere, for example by bubbling an inert gas. Specific examples of inert gases include nitrogen, argon, and helium. The bubbling may also serve as stirring; that is, if sufficient contact between the copper particles and the amine compound can be achieved by bubbling the inert gas alone, stirring may not be necessary.

(2) Process (A-2)
Step (A-2) is a step of removing the solution containing the free amine compound and halogen compound not used in forming the first coating layer from the mixture a to obtain intermediate 1 containing first-coated copper particles. That is, this is a step of removing the solution containing the excess amine compound and halogen compound. At this time, it is not necessary to completely remove the excess amine compound and halogen compound; intermediate 1 can be obtained by separation by natural sedimentation or centrifugation, or by filtration. In other words, intermediate 1 contains small amounts of free amine compound, halogen compound, and solvent. While this may proceed directly to the next step (B), it is preferable to remove them. When removing small amounts of free amine compound, halogen compound, and solvent in intermediate 1, a method in which the copper particles on which the first coating layer has been formed are allowed to settle by natural sedimentation, and then the supernatant solution containing the amine compound and halogen compound is removed by decantation or suction with an aspirator is preferred, due to its simplicity of operation. The obtained intermediate 1 exhibits suppressed aggregation of copper particles, and since aggregation of copper particles does not occur, an increase in bulk specific gravity is suppressed compared to the method described in Patent Document 2. Therefore, for example, the efficiency of liquid replacement by decantation can be improved compared to conventional methods.

Furthermore, the precipitate or filtered product after separation may be washed with a solvent capable of dissolving both the amine compound and the aliphatic monocarboxylic acid having 8 to 24 carbon atoms to obtain Intermediate 1. This washing is preferable because it reduces the amount of free amine compound mixed in.

Intermediate 1 may be dried to reduce the amount of solvent contained (the solvent of the solution containing the amine compound and halogen compound), but drying at this stage may oxidize the copper surface, so it is preferable not to dry it, especially by heating.

If a large amount of free amine compound remains in Intermediate 1, the amine compound will form a salt with carbon dioxide or aliphatic monocarboxylic acid in the atmosphere, resulting in impurities that will adversely affect the conductivity of the conductive composition, which is undesirable.

Therefore, the amount of amine compound in intermediate 1, as the total amount of amine compound forming the first coating layer and free amine compound, is preferably 10% by mass or less of the amount of copper particles. It is more preferable to set it to 1.0% by mass or less so as not to affect the formation of the second coating layer of aliphatic monocarboxylic acid. The amount of amine compound in intermediate 1 can be determined by measuring the amount of amine compound in the supernatant liquid, etc., and calculating the difference from the amount of amine compound used in step (A).

Furthermore, the amount of halogen compounds in intermediate 1 is preferably 1% by mass or less of the amount of copper particles, and more preferably 0.01% or less. If a large amount of halogen compounds remains, they may react with the copper oxide remaining on the copper surface to form copper halides, which may interfere with subsequent processing.

(3) Process (B)
Step (B) is a step of immersing the copper particles that have been subjected to step (A) in a solution containing an aliphatic monocarboxylic acid having 8 to 24 carbon atoms. In this embodiment, copper particles that have been subjected to step (A-2) are used. In step (B), a second coating layer of an aliphatic monocarboxylic acid having 8 to 24 carbon atoms can be formed on the first coating layer formed on the surface of the copper particles.

Specifically, an aliphatic monocarboxylic acid solution containing an aliphatic monocarboxylic acid having 8 to 24 carbon atoms is added to the intermediate 1 to form mixture b, and the mixture b is stirred to form a second coating layer of the aliphatic monocarboxylic acid on the first coating layer. Alternatively, mixture b can be formed by adding the intermediate 1 to the aliphatic monocarboxylic acid solution. The stirring method is not particularly limited, and it is sufficient to stir the copper particles on which the first coating layer has been formed so that the aliphatic monocarboxylic acid comes into sufficient contact with the copper particles. A common stirring method using a known stirrer such as a paddle stirrer or line mixer can be used.

Ideally, the amine compound in the first coating layer bonds with the aliphatic monocarboxylic acid to form a second coating layer in which the aliphatic monocarboxylic acid uniformly coats the first coating layer in the form of a monomolecular film. It is preferable to form a good second coating layer that is as close to ideal as possible. Therefore, the mixing ratio of copper particles to aliphatic monocarboxylic acid in step (B) is preferably a ratio that is suitable for forming this good second coating layer.

Specifically, although this depends on the particle size of the copper particles, 0.1 to 50 parts by mass of aliphatic monocarboxylic acid is preferred per 100 parts by mass of copper particles. 0.5 to 20 parts by mass is more preferred in order to prevent free aliphatic monocarboxylic acid from remaining in the surface-coated copper filler. Since the smaller the particle size of the copper particles, the larger the surface area per unit mass, it is preferable to increase the amount of aliphatic monocarboxylic acid mixed with smaller particle sizes.

The solvent used to prepare the solution of aliphatic monocarboxylic acid having 8 to 24 carbon atoms is not particularly limited, as long as it dissolves the aliphatic monocarboxylic acid, has good wettability with the copper particles and the copper particles on which the first coating layer is formed, and does not react with the amine compound and the aliphatic monocarboxylic acid. A solvent that can be easily dried and removed in the drying step (C) described below is preferred.

Preferred solvents include one or more selected from alcohols, ketones, ethers, nitriles, sulfoxides, and pyrrolidones. Specific examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol, tert-amyl alcohol, ethylene glycol, butoxyethanol, methoxyethanol, ethoxyethanol, propylene glycol, propylene glycol monomethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether. Ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone. Ethers include diethyl ether and dibutyl ether. Nitriles include acetonitrile, propionitrile, butyronitrile, and isobutyronitrile. Examples of sulfoxides include dimethyl sulfoxide. Examples of pyrrolidones include 1-methyl-2-pyrrolidone.

The treatment temperature for forming the second coating layer should be at least a temperature at which the coating of the aliphatic monocarboxylic acid proceeds without solidifying the solution; specifically, a temperature in the range of -10 to 80°C is preferred. A temperature in the range of 10 to 60°C is even more preferred in order to further increase the coating rate and prevent the aliphatic monocarboxylic acid that forms the second coating layer from being released.

There are no particular restrictions on the treatment time, but 5 minutes to 10 hours is preferred, as this allows for sufficient coating of the aliphatic monocarboxylic acid and also makes it less likely that components released as a copper-amine compound-aliphatic monocarboxylic acid complex will remain in the surface-coated copper filler. Furthermore, from the perspective of production costs, 5 minutes to 3 hours is even more preferred.

In addition, it is preferable to carry out step (B) in an inert gas atmosphere, for example by bubbling an inert gas, in order to prevent salt formation between the amine compound of the first coating layer or a small amount of free amine compound mixed in and carbon dioxide in the atmosphere, and to prevent oxidation of the copper. Specific examples of inert gases include nitrogen, argon, and helium. The bubbling may also serve as stirring; that is, if the copper particles on which the first coating layer has been formed can be sufficiently contacted with the aliphatic monocarboxylic acid by bubbling the inert gas alone, stirring may not be necessary.

(4) Process (B-2)
Step (B-2) is a step of removing from the mixture b an aliphatic monocarboxylic acid solution (which may contain an amine compound or halogen compound derived from step A-2) containing free aliphatic monocarboxylic acid not used in forming the second coating layer, thereby obtaining intermediate 2 containing copper particles on which first and second coating layers have been formed. At this time, it is not necessary to completely remove the free aliphatic monocarboxylic acid; a small amount may remain (the same applies to the amine compound or halogen compound derived from step A-2, if present). Specifically, intermediate 2 can be obtained by filtration. Known methods can be used as the filtration method, and examples include natural filtration, reduced pressure filtration, and pressure filtration.

(5) Process (C)
Step (C) is a step of drying the intermediate 2 to obtain a surface-coated copper filler.

The method for drying intermediate 2 is not particularly limited, but examples include vacuum drying and freeze-drying. Vacuum drying is preferred from the perspective of production costs, and the drying temperature is preferably 20 to 120°C from the perspective of drying efficiency and suppressing the acid value of copper. The degree of vacuum, drying temperature, and drying time can be determined appropriately depending on the combination of each condition and the type of solvent used, and are preferably conditions that allow drying to the extent that the amount of solvent in the surface-coated copper filler after drying is 1% by mass or less.

In this manner, particulate surface-coated copper filler can be produced. Furthermore, the surface-coated copper filler obtained in this manner contains few agglomerated copper particles, and the copper particles have a first coating layer of an amine compound and a second coating layer of an aliphatic monocarboxylic acid, resulting in oxidation resistance and dispersibility equivalent to conventional copper fillers. Therefore, such surface-coated copper filler is suitable, for example, as a component of the conductive composition described below.

<Conductive composition>
The conductive composition according to an embodiment of the present invention will be described.

The conductive composition contains 10 to 95% by mass of the above-mentioned surface-coated copper filler. The surface-coated copper filler can be any of those described above, and can be selected appropriately depending on the intended use of the conductive composition. When the surface-coated copper filler is a nanoparticle conductive composition, the average particle size of the copper particles used to produce the surface-coated copper filler is preferably 5 to 100 nm. This average particle size is suitable when the conductive composition is in the form of an ink, as described below.

In addition to the surface-coated copper filler described above, the conductive composition may contain various components commonly used in the relevant technical field. Such components include, for example, binders and solvents.

Binders that can be used in conductive compositions include known binders used in metal pastes, etc., such as thermosetting resins that harden when heat is applied, photocurable resins that harden when light is applied, and thermoplastic resins.

Examples of thermosetting resins include epoxy resins, melamine resins, phenolic resins, silicone resins, oxazine resins, urea resins, polyurethane resins, unsaturated polyester resins, vinyl ester resins, xylene resins, acrylic resins, oxetane resins, diallyl phthalate resins, oligoester acrylate resins, bismaleimide triazine resins, and furan resins. Examples of photocurable resins include silicone resins, acrylic resins, imide resins, urethane resins, and oxetane resins.

Furthermore, examples of thermoplastic resins include polyvinyl chloride, polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene copolymer resin, acrylonitrile-styrene copolymer resin, polymethyl methacrylate, polyvinyl alcohol, polyvinylidene chloride, polyethylene terephthalate, polyamide, polyacetal, polycarbonate, polyphenylene ether, polybutylene terephthalate, polyvinylidene fluoride, polysulfone resin, polyethersulfone resin, polyphenylene sulfide resin, polyarylate, apolyamideimide, polyetherimide, polyetheretherketone, polyamide, polyimide, liquid crystal polymer, and polytetrafluoroethylene.

You can use any one of these binders, or you can use a mixture of two or more of them.

Solvents that can be used in the conductive composition are not particularly limited as long as they have good wettability with the surface-coated copper filler. Examples include alcohols, ethers, ketones, nitriles, aromatic compounds, and water. Alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, tert-butanol, 1-pentanol, 2-pentanol, 3-pentanol, tert-amyl alcohol, 1-hexanol, 1-octanol, 2-ethyl-1-hexanol, ethylene glycol, butoxyethanol, methoxyethanol, ethoxyethanol, ethyl carbitol, ethyl carbitol acetate, butyl carbitol, butyl carbitol acetate, propylene glycol, propylene glycol monomethyl ether, propylene glycol monopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, and terpineol. Examples of ethers include acetoxymethoxypropane, phenyl glycidyl ether, and ethylene glycol glycidyl. Examples of ketones include acetone, methyl ethyl ketone, methyl isobutyl ketone, and γ-butyrolactone. Examples of nitriles include acetonitrile, propionitrile, butyronitrile, and isobutyronitrile. Examples of aromatic solvents include benzene, toluene, and xylene. Any one of these solvents may be used, or two or more may be mixed and used.

Specific examples of the form of the conductive composition include a paste in which the surface-coated copper filler described above is dispersed in a binder, and an ink in which the surface-coated copper filler described above is dispersed in a solvent. The content of the binder and solvent described above can be selected depending on the form of the conductive composition.

When the conductive composition is in the form of a paste, the content of binder in the conductive composition is preferably 10 to 400 parts by mass per 100 parts by mass of surface-coated copper filler. When a paste conductive composition is used to form fine wiring, the cured product of the conductive composition needs to have a lower volume resistivity. To achieve a low volume resistivity, it is necessary to increase the content of surface-coated copper filler in the conductive composition and make it easier for the individual particles of the surface-coated copper filler to come into close proximity with each other, so the amount of binder is more preferably 20 to 100 parts by mass.

When the conductive composition is in the form of ink, the content of solvent in the conductive composition is preferably 10 to 600 parts by mass per 100 parts by mass of surface-coated copper filler. This content ratio is suitable for nanoparticle inks in which the surface-coated copper filler is nanoparticles.

The conductive composition may contain various known additives, such as oxide film removers, antioxidants, leveling agents, viscosity modifiers, and dispersants, as needed. Furthermore, if the conductive composition is in the form of a paste, it may contain a solvent as a diluent to the extent that it exhibits paste properties. If the conductive composition is in the form of an ink, it may contain a binder to the extent that it exhibits ink properties.

The conductive composition contains the surface-coated copper filler described above. As described above, this surface-coated copper filler has few agglomerates, which means that when patterns are drawn by screen printing or inkjet printing, clogging of the screen or inkjet ink is suppressed, resulting in excellent printability. Furthermore, the surface-coated copper filler has good oxidation resistance and dispersibility, and its cured product has a volume resistivity equivalent to conventional fillers.

Conductive compositions can be manufactured using known methods depending on their form. Furthermore, conductive compositions can be cured using known methods to produce a cured product. This cured product is obtained by, for example, applying light or heat to the conductive composition. During this process, shrinkage occurs as the solvent evaporates and the binder hardens. This shrinkage brings the copper particles closer together, thereby exhibiting conductivity. Conductivity can also be exhibited by sintering some of the copper particles.

The following examples and comparative examples further illustrate embodiments of the present invention, but the present invention is not limited to these.

The measurement and evaluation methods used in each example and comparative example are shown below.

<Volume ratio of copper particles in treatment solution>
In Examples 1-1 to 1-13 and Comparative Examples 1-2 to 1-5, 20 g of the dispersion after step [A] was filtered using a filter to separate the copper particles and the solution. Subsequently, 5 g of the filtered copper particles and 8 g of the filtered solution were weighed into a 10 mL vial, the lid was closed, and the mixture was thoroughly stirred and then allowed to stand for 30 minutes. When the settling of the copper particles was confirmed after standing, the height of the liquid surface containing the sediment and the height of the sediment were measured. The value obtained by dividing the height of the sediment by the height of the liquid surface including the height of the sediment and multiplying this value by 100 was used as the volume ratio of copper particles in the treatment solution.

The smaller the volume ratio, the better the decantation performance. The evaluation criteria were: 25% or less was considered a pass (○), and more than 25% was considered a fail (×).

<IR Spectroscopic Analysis>
Measuring instrument model: FT/IR-6100 manufactured by JASCO Corporation
Measurement method: ATR method, resolution: 2 cm ⁻¹ , number of accumulations: 80,

<Screen blockage rate after printing>
(Evaluation method)
Using a screen printing machine (MT-320, manufactured by Micro-Tec Co., Ltd.), a screen plate (SUS 200 mesh) on which a 1 cm square solid film pattern was drawn was used to print the conductive compositions obtained in the Examples and Comparative Examples 10 times at 50 mm/sec onto a polyethylene terephthalate (PET) substrate. After printing, the proportion of the conductive composition remaining in the 1 cm square mesh portion of the solid film pattern in the screen plate was evaluated using a micromicroscope.

(Evaluation of blockage rate)
The 1 cm square peta film pattern on the screen plate after printing as described above was observed at 100x magnification using a micromicroscope (Dino-Lite AM4113), and the percentage of the grids filled with paste was visually counted and calculated for all mesh grids in the field of view. The observation was repeated a total of four times, changing the observation location, and the average was calculated.

<Volume resistivity evaluation>
(Sample Preparation)
The conductive compositions obtained in the Examples and Comparative Examples were applied to alkali-free glass using a metal mask in a pattern measuring 1 cm wide x 3 cm long x 30 μm thick. The patterned glass was heated at 150° C. for 15 minutes to produce a cured product. The volume resistivity of the resulting cured product was measured using the following method.

(Measurement method)
The volume resistivity was measured and evaluated in accordance with JIS K 7194.
Measuring instrument model: Resistivity meter MCP-T610 (manufactured by Mitsubishi Chemical Corporation), measurement conditions: 4-probe method,
Probe: ASP, sample size: 50 mm x 50 mm, film thickness: 1 to 30 μm, number of measurements: 5,

<1. Production of surface-coated copper filler>
Surface-coated copper fillers were produced using the components shown in Tables 1 to 4 and under the copper particle treatment conditions shown in Tables 5A and 5B. However, Comparative Example 1-1 is raw copper particles that were not surface-treated.

(Example 1-1)
[Step (A)]
75 g of water, 300 g of ethylenediamine, and 0.2 g of ammonium chloride were placed in a flask, and nitrogen bubbling was performed for 30 minutes while stirring at 25°C. Then, 100 g of copper particles (1400YP, manufactured by Mitsui Mining & Smelting, average particle size: 6 μm) were added, and the temperature was raised to 60°C. Stirring was performed at this temperature for 60 minutes. Stirring was performed using a mechanical stirrer at a rotation speed of 150 rpm. Stirring in subsequent steps was also performed using the same stirring device at the same rotation speed. 20 g of the dispersion after stirring for 60 minutes was sampled, and the volume ratio of copper particles in the aforementioned treatment solution was measured. The results are shown in Table 5A.

[Step (A-2)]

After stopping the stirring and allowing the mixture to stand for 15 minutes, approximately 325 g of the supernatant liquid was removed. Next, 150 g of ethanol was added to the precipitate as a washing solvent, and the mixture was stirred at 30°C for 3 minutes. After stopping the stirring and allowing the mixture to stand for 15 minutes, approximately 150 g of the supernatant liquid was removed, yielding Intermediate 1.

[Step (B)]

150 g of a 10% by mass solution of capric acid in ethanol was added to Intermediate 1, and the mixture was stirred at 30° C. for 30 minutes.
[Step (B-2)]

After stirring was stopped, the ethanol solution of capric acid was removed by filtration under reduced pressure to obtain Intermediate 2. The filtration under reduced pressure was carried out by applying a vacuum to a Kiriyama funnel equipped with 5C filter paper using a diaphragm pump.
[Step (C)]

Intermediate 2 was dried under reduced pressure at 25°C for 6 hours to obtain a surface-coated copper filler. The reduced-pressure drying was carried out by placing Intermediate 2 in a vacuum oven and reducing the pressure in the oven using an oil pump.

The IR spectrum of the surface of the obtained surface-coated copper filler was measured under the above-mentioned conditions, and the results are as follows.
When the ethylenediamine used for coating was measured alone, the N-H bending vibration peak appeared at 1590 cm ^-1 , whereas the N-H bending vibration peak observed for the surface-coated copper filler was shifted to 1562 cm ^-1 , a lower wavenumber. This indicates that ethylenediamine is present in a coordinated state on the copper particle surface. Furthermore, the C=O stretching vibration peak of capric acid was not observed at 1700 cm ^-1 , and the carboxylate anion ( ^-COO- ) peak was observed at 1413 cm ^-1 . This indicates that capric acid is present in a bond with the amine compound through electrostatic interaction.

From the above IR spectrum, it can be determined that the surface-coated copper filler obtained in Example 1-1 has both the ethylenediamine in the first coating layer and the capric acid in the second coating layer chemically bonded to form each coating layer.

(Examples 1-2 to 1-13)
Surface-coated copper fillers were obtained and subjected to IR measurement in the same manner as in Example 1-1, except that the copper particle treatment conditions shown in Table 5A were used. Furthermore, the volume fraction of copper particles in the treatment solution after step (A) was evaluated in the same manner as in Example 1-1. The results are shown in Table 5A. The IR measurement results for the surface-coated copper fillers obtained in Examples 1-2 to 1-13 confirmed that each formed a coating layer in which the amine compound in the first coating layer and the aliphatic monocarboxylic acid in the second coating layer were both chemically bonded.

(Comparative Example 1-2)
[Pretreatment process]
An aqueous ammonium chloride solution was prepared by dissolving 10 g of ammonium chloride in 200 g of water, and 100 g of copper particles (1400YP, manufactured by Mitsui Mining & Smelting Co., Ltd., average particle size: 6 μm) was added to the aqueous ammonium chloride solution. The mixture was stirred at 30° C. for 60 minutes under nitrogen bubbling, and the copper particles were then filtered off using a filter. As a result of this treatment, copper chloride adhered to the surfaces of the copper particles, as described in Patent Document 2.

[Steps (A) to (C)]
A surface-coated copper filler was obtained and subjected to IR measurement in the same manner as in Example 1-1, except that copper particles that had been pretreated as described above were used and the copper particle treatment conditions shown in Table 5B were used. Furthermore, the volume fraction of the copper particles in the treatment solution described above after step (A) was evaluated in the same manner as in Example 1-1. The results are shown in Table 5B. The IR measurement of the surface-coated copper filler obtained in Comparative Example 1-2 confirmed that the amine compound in the first coating layer and the aliphatic monocarboxylic acid in the second coating layer both formed coating layers bonded by chemical bonds.

(Comparative Examples 1-3 to 1-5)
A surface-coated copper filler was obtained and subjected to IR measurement in the same manner as in Example 1-1, except that the copper particle treatment conditions shown in Table 5B were used. Furthermore, the volume fraction of the copper particles in the treatment solution after step (A) was evaluated in the same manner as in Example 1-1. The results of the IR measurement confirmed that the surface-coated copper filler obtained in Comparative Example 1-3 had the predetermined first and second coating layers formed, that the surface-coated copper filler obtained in Comparative Example 1-4 had only a coating layer of an aliphatic monocarboxylic acid formed, and that the surface-coated copper filler obtained in Comparative Example 1-5 had only a coating layer of an amine compound formed.

As shown in Tables 5A and 5B, the sediment ratio for Examples 1-1 to 1-13 was around 20%, earning a passing grade of "○," whereas the sample pretreated in Comparative Example 1-2 had a sediment ratio of 40%, which was bulky and resulted in poor decantation properties.

(Example 2-1)
20 g of the surface-coated copper filler produced in Example 1-1, 5.7 g of resol-type phenolic resin (PL-5208, manufactured by Gunei Chemical Industry Co., Ltd.) as a binder, 0.3 g of N,N'-bis(salicylidene)ethylenediamine as an additive (oxide film remover), and 1.0 g of terpineol as a diluent were mixed. Next, a planetary mixer (ARV-310, manufactured by Thinky Corporation) was used to perform primary mixing by stirring at 1500 rpm for 30 seconds at room temperature. Next, a three-roll mill (EXAKT-M80S, manufactured by Nagase Screen Printing Research Institute Co., Ltd.) was used to perform secondary mixing by passing the mixture five times at room temperature with a roll distance of 5 μm to produce a conductive composition. Using the obtained conductive composition, the blockage rate and volume resistivity of the screen stencil after printing as described above were evaluated. The evaluation results are shown in Table 6A.

(Examples 2-2 to 2-13, Comparative Examples 2-1 to 2-5)
A conductive composition was prepared in the same manner as in Example 2-1, except that the blending ratios shown in Tables 6A and 6B were used, and the blockage rate and volume resistivity of the screen stencil after printing were evaluated. The evaluation results are shown in Tables 6A and 6B.

The following can be seen from Tables 6A and 6B. The conductive compositions of Examples 2-1 to 2-13 all had a blockage rate of 5% or less, confirming that the conductive composition did not clog the screen stencil and had good printability. On the other hand, Comparative Examples 2-1 to 2-5, including the untreated ones, had a high blockage rate on the screen stencil, confirming that clogging occurred. Furthermore, the volume resistivities of Examples 2-1 to 2-13 were equivalent to those of Comparative Example 2-2, which contained the surface-coated copper filler of Comparative Example 1-2 obtained by a method corresponding to the method described in Patent Document 2, confirming that the copper particles had oxidation resistance. On the other hand, Comparative Examples 2-3 to 2-5 had high volume resistivities and poor oxidation resistance.

Claims (3)

(A) a step of immersing copper particles in a solution containing 0.01 to 30 parts by mass of a metal halide or an ammonium halide derivative per 100 parts by mass of an amine compound represented by formula (1);
[In formula (1), m is an integer of 0 to 3, n is an integer of 0 to 2, and when n=0, m is any of 0 to 3, and when n=1 or n=2, m is any of 1 to 3.]
(B) a step of immersing the copper particles that have been subjected to the step (A) in a solution containing an aliphatic monocarboxylic acid having 8 to 24 carbon atoms;
A method for producing a surface-coated copper filler, comprising: 2. The method for producing a surface-coated copper filler according to claim 1, wherein the halogen atom contained in the metal halide or the ammonium halide derivative is a chlorine atom. The method for producing a surface-coated copper filler according to claim 1 or 2, wherein the ammonium halide derivative is selected from ammonium halide derivatives represented by formula (2):
[In formula (2), a is an integer of 0 to 3, b is an integer of 0 to 3, and a+b=3.]